In the formation of a semiconductor wafer, integrated circuit devices such as transistors are first formed at the surface of a semiconductor substrate. Interconnect structures are then formed over the integrated circuit devices. Metal bumps are formed on the surface of the semiconductor chip, so that the integrated circuit devices can be accessed.
In conventional metal bump formation processes, under-bump metallurgy (UBM) layers are first formed. The UBM layers include a titanium layer and a copper seed layer over the titanium layer. Metal bumps are then formed on UBM layers by plating. Undesired portions of UBM layers are then removed by wet etching. Conventionally, the copper seed layer was removed using ammonical (AM) etching, in which alkaline etchants including Cu(NH3)4Cl2, Cu(NH3)2Cl, NH3, and NH4Cl are used. The resulting chemicals including CuO are then cleaned using NH3 and water. The exposed portions of the titanium layer are then etched using an HF solution with a high concentration of 11 percent. It was observed that undercuts were formed under the metal bumps due to the lateral etching of the titanium layer, and the undercuts may extend under the metal bumps by as much as about 10 μm. As a result, the metal bumps may delaminate from the respective metal pads, resulting in a low yield in the metal bump formation process.